Agents are investigating a purported plot to detonate explosives somewhere in the United States, an assistant U.S. attorney general says after three men were arrested in Colorado and New York.

WASHINGTON — The young Afghan immigrant at the center of an expanding investigation holds the key to unlocking the details of an alleged bomb plot that authorities believe is the first Al Qaeda-linked terrorist operation on U.S. soil since Sept. 11, 2001, law enforcement and intelligence officials said Sunday.

New York City police and the FBI moved aggressively to investigate Najibullah Zazi, 24, and some of his alleged associates last week before the men could take significant steps to launch such a plot, officials said. As a result, there is much that the U.S. government doesn't know about the men's intentions, or their possible connections to a wider network of militants here and overseas, especially in Pakistan, those officials acknowledged.

"You can't make a definitive statement about how big this is because it's unclear how much more is going to come out," said a law enforcement official involved in the inquiry. Like others, the official spoke on condition of anonymity, citing the urgency and secrecy of the investigation and the search for more suspects.

But the official added: "There is a lot more to come. This isn't over by any means."

Authorities are sifting through evidence from raids at numerous locations in New York and Colorado, including Zazi's laptop and other computers, cellphones and other electronic devices. And Zazi, an airport shuttle bus driver living in the Denver suburb of Aurora, had been cooperating with the FBI and other authorities, FBI affidavits unsealed Sunday indicate.

Zazi was arrested in Aurora late Saturday after undergoing three days of questioning by the FBI but refusing to submit to a fourth day. Also arrested were his father, Mohammed Wali Zazi, 53, of Aurora, and Ahmad Wais Afzali, 37, a Queens imam described in court documents as having been an informant for the New York City Police Department. All of them were born in Afghanistan and are living legally in the United States.

The three were charged with deliberately making false statements to federal agents investigating the alleged terrorist plot, a charge that carries a maximum penalty of eight years in prison. All are to appear in court today.

In announcing the charges early Sunday morning, Assistant U.S. Atty. Gen. David Kris said agents were working around the clock, domestically and internationally, in "an ongoing and fast-paced investigation."

Though Kris acknowledged that the Justice Department had "no specific information regarding the timing, location or target of any planned attack," he and other officials said they believed the alleged plot focused on detonating explosives somewhere in the U.S.

Some media reports have quoted authorities as saying that the target may have been a sports arena, mass transit hub or other "soft" civilian target with huge crowds.

But Zazi's potential role remained murky Sunday.

In public comments before his arrest, he said he was a hard-working and devout Muslim who loved America.

Authorities alleged in court documents unsealed Sunday that they had found handwritten formulas and instructions for making bombs, detonators and fuses during a search of Zazi's computer. That occurred sometime after he was stopped Sept. 10 in what authorities said was a random drug check while driving on New York's George Washington Bridge, on a trip from Colorado.

Someone using those diagrams could make bombs of the same size and type as those used in the deadly 2005 attacks on London's transit system, officials said.

Zazi said he might have accidentally downloaded the bomb materials as part of a religious book. But authorities said in charging documents that the handwriting was consistent with Zazi's, and that the materials had been e-mailed between Internet accounts linked to him, including one that originated in Pakistan.

The affidavits say Zazi denied that he had seen or written the handwritten notes on his computer. Agents also allegedly found Zazi's fingerprints on a scale and double-A batteries seized during a raid in Queens on Sept. 14.

The FBI affidavit raises questions about Afzali's connections to Zazi and other men under suspicion. It suggests that the FBI believes he tipped off Zazi to the investigation after being brought to NYPD offices and shown pictures of Zazi "and others," based on an intercepted phone call between the men last week.

Afzali's attorney, Ronald Kuby, told the Associated Press on Sunday that his client had cooperated with authorities, giving them a DNA sample and letting them search his home after the FBI said it was "frantic for any information about Zazi."